ARM: dts: msm: Specify non-default data receive mtu for MSM FERRUM

Increasing the downlink MTU to 4k shows a 14% improvement in end-to-end
bidirectional throughput when the USB link to a host PC is bandwidth
limited.

Change-Id: I8dfa87690a6e3d95482f40d72fd81ccb0dd9a0b3
Signed-off-by: Arun Kumar Neelakantam <aneela@codeaurora.org>